The present invention relates to a circuit and a method for detecting load impedance.
It is known that in some situations it is useful to detect whether the load impedance of an amplifier is higher or lower than a preset threshold value.
For example, audio amplifiers may need to drive different load impedances and therefore to modify parameters of the system according to the type of impedance of the applied load.
If different connectors for active loudspeakers and for passive loads are used, impedance detection with automatic gain adjustment would save a connector and would allow the user to insert the jack without looking for the correct output.
The simplest way to detect an impedance is to apply Ohm""s law in steady-state conditions, measuring the voltage drop across the impedance while a known DC current is injected.
The main drawback of this solution is that the normal operation of the device must be interrupted in order to perform load impedance detection.
For example, detecting the impedance of the load of an audio amplifier can be performed during the power-on step; the drawback of this solution is the fact that if the impedance is switched during normal operations, detection cannot be performed.
In this case, periodic detections are not possible due to the clicking noise that would be generated.
Effective load impedance detection must be performed in real time, when a random signal is present at the output of the amplifier, so that switching of speakers from active to passive can be performed while the amplifier is operating.
Due to reactive components of the impedance which are always present in speakers (self-inductance of the coil and coupling capacitor), current phase offsets must be considered. In order to detect load impedance, known solutions use, as mentioned, Ohm""s law in its general form, V=Z*I. This can be done by means of measurements of the RMS (Root Mean Square) type, by means of integrations performed on the waveform of the output current, or by comparing the output peak voltage with the peak current stored in the analog memory.
These solutions entail complex and accurate analog circuits and the use of a capacitor as an integrator or as an analog memory.
The disclosed embodiment of the present invention provides a device for detecting load impedance to enable impedance detection during the normal operation of the device, in real time.
More particularly, a circuit for detecting load impedance which is circuitally simpler than conventional circuits is provided.
In one embodiment, the circuit for detecting load impedance uses digital memory cells and can be fully integrated. The circuit for detecting load impedance is highly reliable, relatively easy to manufacture and at competitive costs.
The foregoing are achieved by a device for detecting load impedance that includes an analog circuit portion for detecting the impedance value of a load, and a digital circuit portion, which is adapted to provide load impedance type information.
A method for detecting the impedance of a load in real time is also provided, including:
reading across a sense resistor a voltage that is proportional to the output current of an amplifier delivered on a reactive load;
comparing the output voltage of the amplifier with a threshold voltage;
on the basis of the value of the comparison, generating a logic signal for indicating the low- or high-impedance type of the load; and
storing the logic signal at rising and falling fronts of the output voltage of the amplifier in order to detect a low-impedance condition with respectively capacitive or inductive reactive loads.